Carotid artery plaque is known to be an important factor that induces cerebral infarction. Carotid artery plaque is an attached substance that is formed on the inner walls of the carotid arteries by blood cholesterol, neutral fats and the like. In the case this attached substance separates from an inner wall of a carotid artery and obstructs a portion of an intracerebral blood vessel, it can cause a serious disorder such as cerebral infarction. Consequently, known methods used for early diagnosis of the status of the inner walls of blood vessels include diagnostic methods using echo ultrasound from outside the body, and diagnostic methods comprising inserting a catheter having a measuring probe attached thereto into a blood vessel.
There are several types of the above-mentioned plaque, including those having a lipid core composed of cholesterol and atheromas formed from leukocytes and their remains. In the case an atheroma, which is one type of plaque, is ruptured due to some form of pressure or separates from a blood vessel wall, serious disorders such as cerebral infarction are known to be induced at a high probability. Therefore, it is desirable to identify the type of plaque in order to more accurately diagnose the status of blood vessel wall blood vessel walls, and in response to this desire, plaque identification methods have been examined in the manner of those described in Patent Documents 1 and 2, for example. For example, Patent Document 1 discloses a method of inferring the compositions of plaque from the hardness of the plaque determined on the basis of changes in shape caused by pulsation by continuously observing the plaque with a measuring probe inserted into a blood vessel. In addition, Patent Document 2 discloses a method of inferring the composition of plaque by illuminating light components having two or more different types of properties onto a measured portion, receiving the reflected/scattered light from the measured portion, creating a phantom of each optical plaque, and comparing measurement results for the plaque obtained by echo ultrasound with this phantom.